In Patent Literature 1, an invention relating to a wavelength selective switch is disclosed. This wavelength selective switch includes a plurality of light input/output units, a light dispersion means, a light condensing element, and a light deflection element array. The plurality of light input/output units are arranged in an array shape in a first direction and includes a light input unit and a light output unit. The light dispersion means separates wavelength-multiplexed light input from an input unit into wavelength components. The light condensing element condenses the wavelength components after the separation toward the light deflection element array. The light deflection element array deflects the wavelength components in the first direction so that the wavelength components condensed by the light condensing element are switched to a desired output unit. The input/output units are divided into m (m is an integer) groups, and m light deflection element arrays are arranged to correspond to the in groups of the input/output units in the first direction. The light dispersion means is provided in common for the m groups of the input/output units.